Fuel cell using in the cathode compartment and optionally in the anode compartment oxydoreductase enzymes

ABSTRACT

A proton exchange membrane fuel cell comprises an cathodic compartment including a cathode, an oxidant consisting of oxygen and at least one enzyme catalyst, an anodic compartment comprising an anode, a fuel and at least one catalyst. The anodic and cathodic compartments are arranged at either end of the membrane. The cell is characterized in that the enzyme catalyst of the anodic compartment is an oxidoreductase type enzyme capable of catalyzing the reduction of oxygen into hydrogen peroxide and the hydrogen peroxide is a direct receptor of the electrons from the cathode.

FIELD OF THE INVENTION

The present invention relates to a proton exchange membrane fuel cell,using oxidoreductase type enzymes in the cathodic and possibly anodiccompartments.

Therefore, the general field of the invention is that of proton exchangemembrane fuel cells.

STATE OF THE RELATED ART

A fuel cell generally comprises a stack of elementary cells, whereinelectrochemical reactions take place with two reagents which areintroduced continuously. The fuel, such as hydrogen, for cells operatingwith hydrogen/oxygen mixtures or methanol for cells operating withmethanol/oxygen mixtures, and ethanol for cells operating withethanol/oxygen mixtures, is placed in contact with the anode, while theoxidant, generally oxygen, is placed in contact with the cathode. Theanode and the cathode are separated by an ion exchange membrane typeelectrolyte. The electrochemical reactions, the energy of which isconverted into electrical energy, are split into two half-reactions:

-   -   oxidation of the fuel, occurring an the anode/electrolyte        interface producing, in the case of H+ proton hydrogen cells,        which pass through the electrolyte in the direction of the        cathode, and producing electrons, which join the external        circuit, in order to participate in the electrical energy        production;    -   reduction of the oxidant, occurring at the electrolyte/cathode        interface, with water production, in the case of hydrogen/oxygen        cells.

These two reactions have slow kinetics, resulting in the implementationof mineral catalysts, such as platinum-based metallic catalysts, on theanodes and cathodes, to increase the kinetics of these reactions.

However, such catalysts are less effective at low temperatures, whichmay lead to cell start-up problems and induce an overload of thecatalyst electrodes, in order to accelerate said start-up. In addition,these catalysts, essentially based on inert metals, are costly productsand represent potential environmental pollutants.

Finally, these mineral catalysts carry out very satisfactory activationof the anodic reaction, while the cathodic reaction still remainsdifficult to catalyse by this means. For this reason, the cathodicreaction represents a limiting step to the correct operation of a fuelcell.

At the present time, research relates, particularly with respect to gasdiffusion cells, to new arrangements or combinations of existingcatalysts, in order to increase the reactivity of these catalysts. It isspecified that the term gas diffusion cell heretofore and hereafterrefers to a cell for which the oxidant and the fuel supply theirrespective compartments directly in the form of a gas flow.

In the case of cells operating in an aqueous medium, the researchrelates to the improvement of the anodic and cathodic kinetics throughthe use of whole organisms such as bacteria, or through the use ofcomplex enzyme systems grafted onto electrodes, for example made ofplatinum or graphite. The term cell operating in an aqueous mediumheretofore and hereafter refers to a cell wherein the anodic andcathodic compartments are filled with water, the oxidant and fuel thusarriving at its respective compartments in dissolved form.

In this way, the document [1]: Journal of Electroanalytical Chemistry464 (1999), pp 110-117, describes the use of a laccase type enzyme,intended to catalyse the reduction of oxygen into water in the cathodiccompartment of a fuel cell operating in an aqueous medium. This documentspecifies that the use of a laccase alone to carry out oxygen reductiondoes not improve the current densities of the cell, in that the electrontransfer between the cathode and enzyme is carried out according to veryslow kinetics. In order to overcome this drawback, the inventors used anelectrochemical mediator dissolved in the electrolyte, which carries outa rapid transfer of the electrons from the cathode to the active site ofthe laccase. This mediator is2-2′-azinobis(3-ethylbenzothiazoline-6-sulphonate). However, thismediator is not only costly but is also degraded prematurely due to freeradical production, during the transfer of electrons between the cathodeand the active site of the enzyme, via said mediator.

The document [2]: New J. Chem., 1999, pp. 481-487, describes fuel cellsusing glucose as a fuel and cumene peroxide as an oxidant and involvingelectrode surfaces functionalised by a suitable enzyme system. In thisway, at the anodic end, the inventors grafted, on the surface of theanode in contact with the glucose, a monolayer comprising an enzymesystem consisting of an electrochemical mediator referenced MB⁺associated with a flavoprotein type coenzyme, such as FAD, in turnassociated with a glucose oxidase Gox. In this way, the glucose isoxidised, under the effect of glucose oxidase Gox into gluconic acid.The electrons and protons produced are transferred successively to theglucose oxidase associated with the FAD coenzyme to give a Gox-FADH₂type reduced system, followed by the mediator to give MBH₂, whichfinally transfers the electrons to the external circuit, in thedirection of the cathodic compartment. Similarly, at the cathodic end,the inventors grafted on the surface of the cathode in contact with thecumene peroxide a monolayer consisting of a microperoxidase intended tocarry out the reduction of the cumene peroxide.

However, the functionalised surfaces of these electrodes are unstableand difficult to use in industrial environments. In addition, the use ofcumene peroxide as an oxidant cannot be envisaged at a large scale.

The embodiments of the prior art all involve one or more of thefollowing drawbacks:

-   -   they require the use of a large quantity of mineral catalysts,        to catalyse the cathodic reaction;    -   they require, when an enzyme is used, complex functionalisation        of the surface of the cathode, whereto the enzyme must be fixed        to accept the electrons from said cathode.

DESCRIPTION OF THE INVENTION

Therefore, the aim of the present invention is to offer a fuel cell notinvolving the above mentioned drawbacks.

In this way, the inventors of the present invention discovered,surprisingly, that by using a specific enzyme catalyst in the cathodiccompartment of a fuel cell, it was possible to no longer need to usefunctionalisation of the surface of the cathodes, and also to limit, oreven completely eliminate, the use of mineral catalysts from thecathodic reaction.

To this end, the invention relates to a proton exchange membrane fuelcell comprising:

-   -   a cathodic compartment comprising a cathode, an oxidant        consisting of oxygen and at least one enzyme catalyst;    -   an anodic compartment comprising an anode, a fuel and at least        one catalyst, said anodic and cathodic compartments being        arranged at either end of said membrane, said cell being        characterised in that said enzyme catalyst of the cathodic        compartment is an oxidoreductase type enzyme, said enzyme being        capable of catalysing the oxidation of a suitable substrate and        the reduction of oxygen into hydrogen peroxide, said hydrogen        peroxide fulfilling the role of direct acceptor of the electrons        from the cathode.

It is specified that, according to the invention, the termoxidoreductase refers to an enzyme capable of catalysing an oxidationreaction of a first substrate (referred to as a suitable substratewithin the scope of the invention) and a reduction reaction of a secondsubstrate (consisting of oxygen within the scope of this invention).

As mentioned above, the oxidoreductase type enzyme catalyst incorporatedin the cathodic compartment catalyses the oxidation reaction of asuitable substrate and the reduction reaction of oxygen into hydrogenperoxide, said hydrogen peroxide being capable of accepting electronsfrom the cathode directly without requiring the use, for example, of anyelectrochemical mediator.

Unlike the embodiments of the prior art, wherein the enzyme catalystspresent in the cathodic compartment helped improve the electron transferkinetics between the cathode and the oxidant, the oxidoreductase typeenzyme catalyst according to the invention takes part in the hydrogenperoxide production reaction (corresponding within the scope of theinvention to the oxidant of the cathodic reaction), said hydrogenperoxide taking part directly in the cathodic reaction by accepting theelectrons from the cathode to be reduced to water. Given that thespecific enzyme catalyst according to the invention is no longerinvolved in the cathode electron acceptance mechanism, this makes itpossible to simplify the design of said cathode greatly with respect toprior embodiments. In this way, it is no longer necessary to createelectron bonds between the cathode and the enzyme, the creation of suchbonds requiring perfect control of the surface condition of the cathodeand operating conditions (type of electrolyte, for example) to enablethe adsorption, for example, of the enzyme on the surface of thecathode.

In addition, the enzyme catalysts, according to the invention, favourcatalysis of the reduction of oxygen into hydrogen peroxide andoxidation of a suitable substrate at ambient temperature, whichfacilitates the start-up of the fuel cell. Finally, the catalysis of theabovementioned reactions, induced by enzyme catalysts according to theinvention, decreases from a certain temperature threshold. In this way,when a temperature must not be exceeded, it is possible to select asuitable oxidoreductase enzyme, liable to react in a more limited mannerat a given temperature. The implementation of the present invention maythus make it possible to obtain intrinsic safety of the cell, by using agiven enzyme.

The enzyme catalysis, according to the invention, is also perfectlyadjustable. In fact, the abovementioned reactions will only becatalysed, if the suitable substrate and oxygen are added into thecathodic compartment, the suitable substrate corresponding to glucose,when oxidoreductase corresponds to glucose oxidase. It is thenenvisageable to adjust this addition according to the requirements ofthe user of the cell.

Finally, using enzymes, which are not involved in the acceptance of theelectrons from the cathode, may make it possible to replace theelectrodes conventionally used in prior embodiments (such as electrodesmade of graphite or inert metals, such as platinum and gold) byelectrodes made of industrial materials or alloys such as stainlesssteels, aluminium, nickel or titanium alloys or conductive polymermaterials. Preferentially, the cathode according to the invention ismade of stainless steel.

In addition, the enzyme catalysts according to the invention offer theadvantage of being inexpensive and not degrading prematurely.

As mentioned above, the enzymes, according to the invention, capable ofreducing dioxygen are oxidoreductases, which, according to the currentnomenclature, are identified by an EC number of the type EC 1.X.3.Y,where 1 refers to the oxidoreductase class, X characterises the electrondonor substrate, 3 refers to oxygen as the electron acceptor substrate,Y specifically refers to an enzyme, which is included in the subclassdefined by the above three numbers. It is understood that, according tothe invention, these enzymes should catalyse the reduction of oxygeninto hydrogen peroxide.

In addition to carrying out reduction of oxygen into hydrogen peroxide,it is possible to envisage, according to the invention, oxidoreductasetype enzymes also capable, by means of a reaction with said suitablesubstrate, of inducing an acidification of the cathodic compartment,said acidification facilitating the reduction of oxygen into hydrogenperoxide and subsequently the reduction of hydrogen peroxide into wateron the surface of the cathode. This phenomenon is particularlyadvantageous when the cathode is made of stainless steel, in that theacidification of the cathodic compartment may make it possible toactivate the surface of the cathode, with a view to facilitating thereduction of hydrogen peroxide into water.

For example, the oxidoreductase type enzymes may be selected from thegroup consisting of galactose oxidase, glucose oxidase, pyruvateoxidase, glutamate oxidase, alcohol oxidases. For the above mentionedenzymes, it is understood that the suitable substrates are respectivelygalactose for galactose oxidase, glucose for glucose oxidase, pyruvatefor pyruvate oxidase, glutamate for glutamate oxidase, an alcohol foralcohol oxidases.

The EC numbers of these enzymes are respectively EC 1.1.3.4 for glucoseoxidase, EC 1.1.3.9 for galactose oxidase, EC 1.2.3.3 for pyruvateoxidase, EC 1.4.3.7 for glutamate oxidase.

Preferentially, the oxidoreductase type enzyme used in the cathodiccompartment is glucose oxidase.

For glucose oxidase, the active site of said oxidase induces theoxidation of the glucose substrate into glucono-1,4-lactone which issubsequently hydrolysed into gluconic acid. Concomitantly, the saidactive site induces the reduction of oxygen into hydrogen peroxide whichis subsequently reduced into water by the electrons arriving from thecathode, the acidification of the cathodic compartment by gluconic acidfavouring these two successive reductions.

Glucose oxidase also offers the advantage of making it possible to forman intrinsic safety device of the cell, in that glucose oxidase is nolonger active at around 70° C. Therefore, it is of particular interestfor applications involving a cell, according to the present invention,where this temperature must not be exceeded.

For the anodic compartment, the catalysis of the anodic reaction (i.e.the oxidation reaction of a fuel) may be carried out using any type ofcatalysts, including metallic catalysts.

However, very advantageously, the catalyst of the anodic compartment is,according to the invention, an enzyme capable of catalysing theoxidation of a suitable substrate, said substrate acting as a fuel.

Preferentially, the enzyme of the anodic compartment is also capable, bymeans of a reaction with said substrate, of carrying out acidificationof the anodic compartment.

In the same way as for the cathodic compartment, using an enzyme as acatalyst makes it possible to limit, or even eliminate, the mineralcatalyst load.

In this way, the enzyme of the anodic compartment may be selected fromthe group consisting of hydrogenases, glucose oxidase, galactoseoxidase, alcohol oxidases. It is understood that the substrates for thelist of enzymes mentioned are respectively hydrogen, glucose, galactose,suitable alcohols.

It should be noted that some of these enzymes, such as glucose oxidase,consume dioxygen in order to function. For this reason, it will not benecessary to purge said compartment of its dioxygen, as is the case whenhydrogen serves as the fuel.

Advantageously, according to the invention, the enzyme of the anodiccompartment is glucose oxidase and the fuel glucose.

The oxidation of glucose by this enzyme produces glucono-1,4-lactone,which is hydrolysed into gluconic acid, thus releasing protons, requiredfor the operation of the cell. These protons are carried, in particular,in the direction of the cathodic compartment via the proton exchangemembrane.

It is specified that, given that the enzyme of the anodic compartmentaccording to the invention plays a direct role in the anodic reaction,i.e. in the transfer of electrons from the fuel to the anode, thisenzyme is advantageously immobilised on the surface of the anode. Theenzyme may be immobilised by conventional means known to those skilledin the art such as simple adsorption, a co-cross-linking reaction withglutaraldehyde, inclusion in Nafion type polymer membranes or insurfactant layers deposited on the anode, electrostatic interactionswith polyions adsorbed on the surface of the anode, grafting by covalentbonding.

The use of such a system also offers the advantage of no longerrequiring the use of hydrogen, which may pose supply and safetyproblems.

The present invention may be applied equally well to gas diffusion cellsand to cells operating in an aqueous medium.

With respect to the introduction of enzyme catalysts and substrates inthe electrode compartments (i.e. anodic and cathodic), variousalternatives may be envisaged.

According to a first alternative, the enzyme(s) of the anodic and/orcathodic compartment and the substrate(s) may be introduced continuouslyor discontinuously into the respective compartments during the operationof the cell. For example, for a cell operating by means of gasdiffusion, the enzymes may be introduced in the form of aerosols withsuitable substrates.

According to a second alternative, the enzyme(s) of the anodic and/orcathodic compartment are adsorbed on the anode and/or on the cathode.

Finally, for cells operating in aqueous media, the enzyme(s) are,according to a particular embodiment, introduced directly into theaqueous medium of the anodic and/or cathodic compartment, during theassembly of the cell.

The invention will now be described with reference to the examples,given for illustrative and not limitative purposes.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 represents the diagram of a cell, according to the invention,operating in an aqueous environment.

FIG. 2 represents the diagram of a gas diffusion cell, according to theinvention, with catalysis of the cathodic reaction by glucose oxidase.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS Example 1

The cell used, in this example, is a cell operating in an aqueousenvironment and is represented in FIG. 1.

This cell comprises respectively a cathodic compartment 1 and an anodiccompartment 2 separated by a proton exchange membrane 4. The electrodesare respectively a platinum grid for the anode 6 and a stainless steelplate for the cathode 8.

A water flow 10 from tanks 11, said flows being previously saturatedwith dihydrogen for the flow intended for the anodic compartment 2 andwith dioxygen for the flow intended for the cathodic compartment 1,arrives at each compartment.

According to this example, the saturation of the water flow withdihydrogen or dioxygen is carried out by bubbling the respective gasesin the water of the tanks 11, said gases being routed to the tanks 11via a dioxygen inlet 14 at the cathodic end and by a dihydrogen inlet 12at the anodic end.

In addition, according to this example, glucose and glucose oxidase areintroduced into the tank 11 at the cathodic end via an inlet 16positioned in the vicinity of the oxygen inlet 14. On each tank 11,outlets 18 are provided to replenish the aqueous medium.

Under such conditions and for glucose concentrations of 24.4 mM andglucose oxidase concentrations of 2.0 U/ml, the output supplied is 70 to280 times greater in the presence of glucose oxidase than with none, fordifferent electrical resistance values are presented in table 1 below.

TABLE 1 Resistance 1 Ω 10 Ω 100 Ω 1000 Ω Ratio 280 260 170 70 (Outputwith enzyme/output without enzyme)

Example 2

The cell used in this example is similar to the cell in FIG. 1 describedabove.

However, the cell according to this example differs on the followingpoints:

-   -   glucose replaces dihydrogen as the fuel, at the anodic end;    -   the anode 6 is made of stainless steel    -   glucose oxidase is grafted directly onto the surface of the        anode 6.

In this way, according to this example, it is no longer necessary torequire the use of an inert material for the composition of the anode,due to the fact that the catalysis at the anode end is no longer carriedout by metal catalysts and dihydrogen is no longer used, but glucose,which is easier to handle.

Example 3

The cell used, in this example, is represented in FIG. 2.

This cell comprises respectively an anodic compartment 20 and a cathodiccompartment 22 separated by a proton exchange membrane 24, the anode andthe cathode being made of graphite. In both compartments, platinum (1mg/cm²) is used. The anodic compartment 20 is equipped with a dihydrogeninlet 26 which passes prior to its introduction into the compartmentthrough a humidifier device 28, while the cathodic compartment isequipped with a dioxygen inlet 30 passing through a humidifier device 28and laterally a glucose and glucose oxidase inlet 32. In this way, theenzyme and its substrate are injected in operation, which particularlymakes it possible to adjust the intensity of the catalysis of thecathodic reaction, therefore the operation of the cell, by modulatingthe quantity of glucose injected. The catalysis of the anodic reactionis carried out by the platinum. In each compartment, outlets 34 areprovided to replenish the oxidant and fuel, enzymes and substrates.

In the embodiment, the glucose and glucose oxidase inlet 32 is used toinject 1 ml of a solution containing 20 mM of glucose and the glucoseoxidase content indicated in table 2, at the start of operation of thecell.

TABLE 2 Glucose oxidase 0 1 10 100 concentration unit/l unit/l units/lunits/l Ratio (Output with 1 1.15 1.15 1.28 enzyme/output withoutenzyme)

It is observed that, under such conditions, the addition of enzymesmakes it possible to increase the catalysis of the cathodic reaction andtherefore the output of the cell from 15% (with 1 or 10 units/l) to 28%(with 100 units/l).

This result is remarkable in that it demonstrates that the addition ofenzyme makes it possible to improve the performances of a cell which,however, uses a platinum catalyst at standard quantities for commercialcells.

1. Proton exchange membrane fuel cell comprising: a cathodic compartmentcomprising a cathode, an oxidant consisting of oxygen and at least oneenzyme catalyst; an anodic compartment comprising an anode, a fuel andat least one catalyst, said anodic and cathodic compartments beingarranged at either end of said membrane, said cell being characterizedin that said enzyme catalyst of the cathodic compartment is anoxidoreductase enzyme which is glucose oxidase, said enzyme beingcapable of catalyzing the oxidation of suitable substrate and thereduction of oxygen into hydrogen peroxide, said hydrogen peroxidefulfilling the role of direct acceptor of the electrons from thecathode.
 2. Fuel cell according to claim 1, characterized in that theoxidoreductase type enzyme is also capable, by means of a reaction withsaid suitable substrate, of carrying out acidification of the cathodiccompartment.
 3. Fuel cell according to claim 1, characterized in thatthe cathode is made of a material selected from the group consisting ofstainless steels, aluminium, nickel or titanium alloys, conductivepolymers.
 4. Fuel cell according to claim 1, characterized in that thecatalyst of the anodic compartment is an enzyme capable of catalyzingthe oxidation of a suitable substrate, said substrate serving as a fuel.5. Fuel cell according to claim 4, characterized in that the enzyme ofthe anodic compartment is also capable of carrying out, by means of areaction with said suitable substrate, acidification of the anodiccompartment.
 6. Fuel cell according to claim 4, characterized in thatthe enzyme of the anodic compartment is selected from the groupconsisting of hydrogenases, glucose oxidase, galactose oxidase, alcoholoxidases.
 7. Fuel cell according to claim 4, characterized in that theenzyme of the anodic compartment is glucose oxidase and the fuel isglucose.
 8. Fuel cell according to claim 1 or claim 4, characterized inthat the enzyme(s) of the anodic and/or cathodic compartment and thesubstrate(s) are introduced continuously or discontinuously into theirrespective compartments during the operation of the cell.
 9. Fuel cellaccording to claim 1 or claim 4, characterized in that the enzyme(s) ofthe anodic and/or cathodic compartment are adsorbed on the anode and/oron the cathode.
 10. Fuel cell according to claim 1, characterized inthat the cell is a gas diffusion cell.
 11. Fuel cell according to claim1, characterized in that the cell is a cell operating in an aqueousmedium.
 12. Fuel cell according to claim 11, characterized in thatenzyme(s) are introduced directly into the aqueous medium of the anodicand/or cathodic compartment, during the assembly of the cell.